Method of continuously heat treating aluminum strip



METHOD OF CONTINUOUSLY HEAT TREATING ALUMINUM STRIP Filed Feb. 25, 1950May 19, 1959 M. D. STONE ETAL 5 Sheet-Sheei 1 INVENTORS means 0. s'rousuosEPH LGREENBERGER May 19, I959 M. D. STONE ETAL 2,887,422

METHOD OF CONTINUOUSLY HEAT TREATING ALUMINUM STRIP Filed Feb. 25, 19505 Shuts-Sheet 2 INVENTOR5 MQRR\S D. STONE JOSEPH LGREENBERGER BY/ 2 zMay 1 9 1959 .M. D. STONE ETAL 2,887,422

msmon 0 CQNTINUOUSLY HEAT TREATING ALUMINUM STRIP Filed Feb. 25, 1950 5Sheet's -Sheet :s

MQRR\$ STONE. I JOSEPH GREENBERGER I IINVENTORS May 19, 1959 M. D.STONE; ETAL 2,887,422

METHOD OF commuousw HEAT TREATING ALUMINUM STRIP Filed Feb. 25, 1950 fas Sheets-Sheet 4 y 1959 M. D. STONE ETAL 2,887,422

METHOD OF CONTINUOUSLY HEAT TREATING ALUMINUM STRIP MORR\S D. STONE.JOSEPH LGREENBERGER BY wan;

United States Patent METHOD OF CONTINUOUSLY HEAT TREATING ALUMINUM STRIPMorris D. Stone and Joseph I. GreenbergenPittsburgh,

Pa., and Marcel Lamo'urdedieu, Neschers, France, assignors to UnitedEngineerin and Foundry Company, Pittsburgh, Pa., a corporation ofPennsylvania Application February 25, 1950, Serial No. 146,376

6 Claims. (Cl. 148-2191) This invention relates to a method of heattreating metallic strip and, in particular, for the heat treatmentwithin the solution heat treating temperature range of aluminum alloy,as well as the various other continuous strip alloy materials havingsimilar metallurgical characteristics, which is followed by rapidquenching to a temperature sufliciently below the heat treating range toinitially retain the high temperature metallurgical structure and, asrequired for the particular alloy involved, aging at either normal orelevated temperatures. The solution heat treating temperature range of aparticular alloy is the range within which complete solid solution ofthe alloy constituents occurs within the parent metal. For example, inthe case of the duralumin alloy, known commercially as 248, one of .theprincipal alloying elements copper may be present in percentages varyingfrom 3.8% to about 4.9%, and when the alloy is heated to a temperaturewithin the solution heat treating range, all of the copper is throwninto solid solution in the aluminum.

For a particular aluminum alloy, the temperatures comprising thesolution heat treating zone or range are readily obtainable fromstandard handbooks and published articles dealing with the physicalandchemical properties of metals and their alloys. The solution heattreating zone of the above referred to commercially designated 24Saluminum alloy, as is well known in the metallurgical art, isapproximately 920 F. plus or minus 10 F.

The present day practice for the solution heat treatment of light gaugealuminum alloys involves suspending groups of sheets of the materialwithin a furnace or heated salt bath at a temperature Within thesolution heat treating range for from twenty to thirty minutes and thenremoving the heated sheets therefrom, quenching rapidly in a cold waterbath, drying, straightening by roller leveling and then, if necessary,stretcher leveling and skin passing for temper as may be'required. Suchprocessing of individual sheet material, however, is quite expensiveinasmuch as considerable manual labor is required, the operation istime-consuming, and the scrap loss is exces s1ve.

By experiment, it has been determined that once aluminum alloy stripmaterial attains the required solution Inasmuch as it has been thepractice of retaining the alloy for a long period of time at thesolution heat treating a temperature before quenching, as opposed to theshort time cycle herein disclosed, previous attempts of continuoussolution heat treating alloy materials were discouraged.

Where the strip may be held at the j .The apparatus herein disclosed isadapted to handle "ice aluminum alloys in strip form suppliedcontinuously from coils introduced into the entry end of the line. Aninduction heatingunit, through which the strip passes for initialheating, is adapted to raise rapidly the temperature of the particularalloy to the necessary solution heat treating temperature or to atemperature closely adjacent thereto at which point additional heat maybe applied from a separate source in order to bring the temperature intothe solution heat treating zone. Within the chambers of a temperatureholding furnace, through which the strip next passes, the initial striptemperature is .maintained by means of heated air supplied from a closedauxiliary air reheating, recirculating system. Suflicient looping of thestrip is permitted in order that the strip will be held within thesolution heat treating temperature zone for a relatively short period oftime extending from a minimum of one half minute to three minutes atrated line speeds. At the delivery end of the holding furnace there isprovided a quenching bath within which the strip is rapidly quenched toachieve the desired metallurgical and physical properties, especiallythat of resistance to corrosion.

With the foregoing in mind, it is one of the objects of this inventionto provide anefiicient, economical method for the solution heat treatingof continuous metal strip.

Another object of this invention is to provide a method adapted to bepracticed for the rapid heating of continuous strip metal to atemperature within the solution heat treating range and for maintainingthe temperature Within the range for a sufficient period of time priorto quenching which will result in a heat treated product havingsatisfactory metallurgical properties.

Still another object of this invention is to provide a heat treatingmethod adapted to be practiced for solution heat treating metal incontinuous strip form by passing the strip through a critical heattreating zone under controlled conditions of temperature, speed andtension.

A further object of this invention is to provide a method of heattreating metalin continuous strip form by which the metal is rapidlyheated to the temperature desired, maintained at that temperature for adesired period of time, and then quickly quenched and recoiled.

These objects, as well as the various other novel features andadvantages of this invention, will become apparent from the followingdescription and accompanying drawings, of which,

Figure l is a side elevation view of a continuous metal strip heattreating line embodying the features of this invention;

Figure 2 is a plan view of the apparatus illustrated in Figure 1;

Figure 3 is a side elevation view of the holding furnace;

Figure 4 is a side elevation view taken from the drive side of theholding furnace;

Figure 5 is a side elevation view showing the heat treating linethreading device;

Figure 6 is a diagram showing the circuit and the controls for theinduction heating unit, and

Figure 7 is an elevation view of a modified form of the invention hereindisclosed.

With referenceto the drawings, particularly Figures 1 and 2, thecontinuous heat treating line comprises a coil box 11, a friction drag12, a snip shear 13, a stitcher or welder 14 and a clamp 15 associatedtherewith, a pair of pinch rolls 16 on either side of :a looping pit 17,a horizontally disposed pinch roll unit 18, induction heating unit 19, astrip temperature retaining or holding furnace 21, a quenching tank 22,a hot air drier 23, a pair of vertical pinch roll units 24 on eitherside of a looping pit 25,. a strip leveler 26, a deflector roll 27, asnip shear 28 and a strip recoiling reel 30. The pinch roll units 16 aredriven by motors 16a, the pinch roll unit 18 by a motor 18a, the pinchroll units 24 by motors 24a, the hot air drier 23 provided with an airheater and blower 23a, the leveler 26 driven by a motor and gearreduction unit 26a and the reel 30 operated by a motor 3001. As thecontrols for the various motors include separate speed adjustment meanssuch as rheostats or the like, which are well known in the art, by whichthe speed of each motor can be initially adjusted to provide for auniform feeding of the strip through the line, it is deemed to beunnecessary to set forth and illusstrate specifically the detailsthereof.

The induction heating unit 19 as shown more specifically in Figure 3,within which the continuous strip material is elevated to the desiredtemperature, includes a frame made up of vertical spaced channels 29 tothe sides of which are welded spaced cross channels 31 and to the topthereof channels 32. Supported by cross members 33 welded to thechannels 29 at the entry end of the furnace is an induction heating unitcoil assembly 34, between the poles of which a suitable channel 35 isprovided through which the strip is passed for continuous heating to thetemperatures desired. On either side of the magnets of the inductionheating unit there are elongated edge shields having slots thereinthrough which the edges of the strip pass. Although a cooling system isprovided, consisting of both a blower and a hydraulic system for forcingair and a cooling fluid through various parts of the coils, the sidesand the framework thereof are open to the air to permit additionalcooling of the coils by direct contact with the atmosphere.

Between the bottom of the induction heating unit 19 and the top of thestrip heat retaining or holding furnace 21, there are three stripstraightening rolls 36, 37 and 38, shown in Figure 4, supported insuitable bearings at the ends of the shafts thereof, adapted to have thestrip pass therebetween for removing the kinks from the material and toprovide a flat surface therefor as it is discharged from the inductionunit and prior to the entry thereof into the holding furnace 21.

The holding furnace 21, as best shown in Figure 3, comprises twochambers or sections 39 and 41 which are in direct communication withone another and which are, on the outer surface thereof, totallyenclosed by means of metal sheets and a suitable heat resistant laggingmaterial 42. At the entry side of the holding furnace there is a narrowopening 43 through which the strip is adapted to pass without permittingan undue amount of heat to escape therefrom. A similar opening 44 isalso provided at the discharge side of the furnace.

Within the furnace section 39 and near the bottom thereof, there is alooping roll 45 which is supported on a shaft 46. The ends of the shaftpass through suitable openings in the sides of the furnace and aremounted in floating bearings 47 attached to one end of a lever 48pivotally mounted on a shaft 49 supported by pedestal bearings 51. Toeach of the floating bearings 47 there is secured at its outer end apiston rod 52 adapted to be extended and withdrawn by a cylinder 53pivotally mounted on a bracket 54. On one end of the shaft 49, Figure 4,there is keyed a sprocket 55 over which passes a chain 56 driven by asprocket 57 mounted on the shaft of a motor 45a. Also on the motor shaftis a second sprocket 59 which is engaged by a chain 61 which passes overa sprocket 62 of a PIV unit 63 whereby a sprocket 64 on the output sideof the unit is rotated. The sprocket 64 is engaged by a continuous chain65 which also engages and drives sprockets 66, 67 and 68 mounted on theshafts of rolls 36, 37 and 38 respectively.

Mounted within the top enclosed portion and between the two rolls 37 and38 closely adjacent to the path of the strip is a strip temperaturemeasuring device 69 adapted to respond to the temperature of the stripas it issues from the induction unit and to influence thevoltageregulator whereby the voltage will be modified to the. extentnecessary to effect the desired amount of 4 heat to be imposed upon thestrip by the induction heating unit. This is not disclosed in completedetail as the construction thereof may take various forms and suitablyconstructed units for this purpose are available on the market.Preferably, such a unit should be one in which no contact is made withthe strip.

Within and at the top of the holding furnace chamber 41 there are twowrapper rolls 71, the shaft ends of which are extended through the wallsof the furnace and supported in suitable bearings 72 secured to crossmembers 73 extending across the furnace wall. Each of the wrapper rolls71. is driven by an electric motor 71a directly connected thereto.Encompassing each of the wrapper rolls 71 for a considerable portion ofthe circumference thereof there is a continuous belt 74 formed of asuitable heat resistant material. The belt 74 is supported by threeidler rolls 75, having shaft ends supported in suitable bearings outsidethe walls. of the furnace, and by an additional idler roll 76 supportedin floating bearings 77 received in: guides on the outside of thefurnace wall. To the floating bearings weights 78 are suspended forurging the bearings downward in order to maintain the continuous beltsin positive contact with the wrapper rolls 71.

At the bottom of the chamber 41 there is a looping roll 79 supported bya shaft 81. This roll is mounted in a similar fashion to the roll 45,which is provided with bearings 82 secured to levers 83 and to the endsof piston rods 84 extending upward from a pair of cylinders 85. Inasmuchas the tension of the strip material passing' around the rolls 45'and 79is to be quite limited, both of the rolls 45 and. 79 can be urged upwardby a sufficient force either to permit only a portion of the weight ofthe rolls and the levers to act upon the strip or to an extentsufficient to remove all tension from the strip with the exception. ofthat induced by the weight of the strip itself.

To the bearings 47 of the roll 45 on the side of the furnace oppositethe drive side there is attached one end of a chain 86 which passes overa sprocket 87 secured to the shaft of a rheostat 88 attached to thefurnace wall. To the opposite end of the chain 86 is secured. a weight89. The rheostat. 88 is connected into the electrical circuit of thefirst wrapper roll motor 71a and motor 45a and as the looper roll 45rises or falls, the speed of the looper roll 45 and the followingwrapper roll 71 will decrease or increase, by reason of the variation ofthe resistance within the circuit, in order to control strip lengthconditions within the first part of the furnace. Similarly, a chain 90is attached to the healing of the looper roll 79 and passed over asprocket 91 secured to the shaft of. a rheostat 92 attached to the wallof the furnace. A weight 93 is suspended from the opposite end of thechain. The rheostat 92 is connected into the electrical circuit of thesecond motor 71a and motor 79a so that as the roll 79 rises or falls,the speed of the looper roll 79 and the second wrapper roll 71 willdecrease or increase, by reason of the variation of the resistancewithin the circuit, to provide the desired strip length control withinthe latter part of the furnace.-

A hot air blowing system is provided for re-circulating and re-heatingthe atmosphere of the holding furnace and includes a motor driven blower94 which draws the atmosphere from the furnace chamber 39 through a duct95 and forces it through an electric resistance heating unit 96 and duct97 into the furnace chamber 41. A thermostat 98 is positioned at thedischarge end of the duct 97 and is connected into the circuit of theresistance heating unit 96 so that the current passing through the unit96 can be accurately controlled for effecting the desired airtemperature.

Below and at the discharge end of the furnace there is a quenching tank22. This tank includes a cold water spray 99, enclosed within a chamber100, through which the strip first passes on issuing from the furnace. Adeflecting roller 101 around the lower portion of which the strippasses, is rotatably mounted between the two lower arms of a forked bellcrank 102 which is pivotally mounted on a shaft 103 supported inbearings 104 at the bottom of the tank. The roller 101 is provided witha sprocket 105 over which passes in driving relationship a chain 106from a sprocket 107 attached to the shaft of a driving motor 101a.Upwardly extending from the shaft 103 is an arm 108 pivotally secured atits upper end to a piston rod 109 extending from and actuated by ahydraulic cylinder 111 for counterbalancing the weight of the roller 101and its assembly so that a controlled strip tension is maintained.

To a pin 112 extending out from the arm 108 is attached one end of achain 113 which passes over a sprocket 114 secured to the shaft of arheostat 115. A weight 116 is suspended from the opposite end of thechain. The rheostat 115 is connected into the electrical circuit of thepinch roll unit driving motor 24a at the entry side of the looping pit25 and that of the driving motor of roller 101, so that as the arm ismoved toward or away from the furnace due to a change in length of thestrip, the speed of the motors 101a and 24a are caused to increase ordecrease, changing the speeds of the roller 101 and the pinch rolls 24respectively, thereby maintaining the desired condition of balance. Thisinsures that the strip will pass below the level of the water in thetank, for additional cooling, without imposing unnecessarytensioning'forces on the strip at the discharge side of the furnace.

At the bottom of each looping pit there are two sets of photocellcontrols. In the first pit 17, a lower set 117 is connected into thecircuit of the pinch roll motor 16a at the entry side of the pit so thatwhen the strip droops so low as to interrupt the beam of light, thefirst pinch roll motor 16a is slowed down slightly in order to permitthe loop to rise. The second set of photocell controls 118 is 'above thefirst set and connected into the same electrical circuit as the first.Should the loop in the pit rise so high as to permit the beam of lightto pass uninterruptedly to the top photocell, then the motor at the pitentry end is speeded up to cause the loop again to grow in the pit. Thephotocell controls 119 and 121 in the second pit 25, function in asimilar fashion and are connected in the circuit of the pinch roll motor24a, at the discharge end of the pit 25, for controlling the speedthereof.

With reference to Figure 6, the control system for providing thenecessary electrical current for the induction heating unit includes acircuit breaker 122, connected to the power line, as shown, and to anauto transformer 123 which, in turn, is connected to an inductionvoltage regulator 124 connected through two busses 125 and 126 to thecoils of the induction heater coil assembly 34 and to a bank ofcapacitors 127. The strip temperature measuring device 69, locatedbeneath the induction coil assembly 34, is connected to a temperatureindicating unit 161 having leads extending therefrom through whichcurrent can be passed for energizing either a forward relay coil 162 ora reverse relay coil 163 depending upon whether the temperature of thestrip is above or below a pre-determined amount. When, for example, thetemperature of the strip tends to become too low, the forward relay coil162 is energized closing a series of contacts F F and F so that a motor164 will rotate in the forward direction, which slowly rotates the rotorof the voltage regulator whereby the energy supplied to the inductionheating unit is increased. When the strip temperature is above apredetermined amount, the reverse relay coil is energized to close thecontacts R R and R causing the motor 164 and the voltage regulator rotorto rotate in the opposite direction to decrease the energy supplied tothe induction heating unit. By proper adjustment of the capacitorcontrols, it is possible at all times to maintain substantially a powerfactor of unity in the electrical system.

For threading the leading end of the strip material through the mostinaccessible portions of the line there is shown in Figure 5 a unithaving two cables which are adapted to engage the strip end at the entryend of the holding furnace and to carry the strip through the furnaceand the remainder of the line to the recoiling reel. Secured to the topof the furnace frame at opposite sides thereof are two tackle blocks 128around the sheaves of which pass separate cables 129 which in turn arepassed over the sheaves of vertically movable tackle blocks 131 havingweights 132 secured thereto. The free end of each cable is passed overtwo sheaves 133 supported by brackets secured to the outside of thefurnace, through the opening 43, around several pairs of sheaves 134within the temperature holding furnace, through the opening 44 at thedischarge end of the furnace, and oversingle sheaves 135, a pair ofsheaves 136, and over additional single sheaves 137 and 138 and throughthe two pinch roll units 24, and finally to the reel 30 where the endsthereof are attached for winding a portion of the cables thereon. Theposition of the tackle blocks shown in Figure 5 is that of maximumcablestorage and is that from which threading of the line takes place. Asuitable strip gripper bar 139 is adapted to be secured to the leadingend of the strip, at the exit end of the induction heating unit and tothe threading cables 129 so that the reel can be rotated for slowlywinding the cables thereon to cause the strip to be drawn through theholding furnace to the strip leveling unit 26. As soon as the threadinghas been completed, the rolls of the pinch roll unit 24 on the exit sideof the looper pit 25 are closed, the gripper bar is detached from thestrip and the cables, and the strip fed through the leveler and shearand up to the reel 30. The cables are unwound from the reel 30 and byvirtue of the weights 132, the tackle blocks 131 move downward so thatthe excess cable will be gathered for storage between the stationary andthe movable blocks. The two cable ends are then detached from the reeland secured preferably to the frame of the roller leveler where theywill be easily accessible for future threading operations.

In the operation of the heat treating line, a coil of strip material tobe processed is placed in the coil box 11 from which a portion of metalis unwrapped and passed by hand through the wiper 12,. the shear 13where the leading end thereof is cropped, through the stitcher or Welder14 and clamp 15, through pinch rolls 16 and looping pit 17, and throughthe horizontal pinch rolls 18, the induction heating unit 19 and betweenthe three straightening rolls 36, 37 and 38. At this point, the gripperbar 39 is secured to the leading end of the strip and the ends of thebar attached to the two cables 129. Since in the threading operation theends of the cables are attached to the reel 30, by rotating the reel,the cables are wrapped thereon and the strip threaded through theholding furnace 21, the quench tank 22, the drier 23, and the two pinchroll units 24 at which point rotation of the reel is interrupted. Thegripper bar 39 is thereupon detached from the cables and the strip fedthrough the leveler and shear and up to the reel. The excess cable isunwound from the reel and gathered between the tackle blocks 128. Theends of the cables are released from the reel and secured preferably tothe frame of the leveler 26.

The leading end of the strip is then detached from the gripper bar 39and passed through the leveler and shear and attached to the reel 30.With the furnace at the required heat, the various motors are energizedand the speed thereof adjusted to provide the necessary solution heattreating conditions. On completion of the uncoiling of the first strip,the trailing end thereof is sheared and movement thereof interrupted inthe stitcher or welder 14 for attachment to the squared leading end of anew coil inserted in the coil box 11. The looping pit at the entry endof the line provides for a continuous supply of strip during the stripjoining operations and the pit 25 permits the strip, as it issues fromthe furnace, to bestored therein during the interval that the stitchedends are cut out of the strip at the shear 28 and the coil removed fromthe reel.

In accordance with Figure 7, illustrating a modification of theinvention herein disclosed, an auxiliary heating means is provided forapplying additional heat to the strip and raising the temperaturethereof to the re quired holding temperature. If continuous strip metalhaving cracks at the edges thereof is heated by induction means, ahigher temperature will be induced in the zones of the cracked edges dueto the concentration of current caused by the crowded divergent path itis required totake. Should the main body of strip having only slightlycracked edges be heated initially to the required temperature, then dueto overheating, an undesirable considerably higher temperature willoccur at the edges. In order to avoid subjecting the strip edges tooverheating, the modified apparatus includes an induction heating unit151, wherein duraluminv strip, for example, is heated to a temperatureof approximately 880 to 900 Fahrenheit which is slightly below thesolution heat treating temperature, and an auxiliary heating unit 152provided with nozzles 153 which are directed downward at an angle withrespect to the strip and through which heated air is passed at a highvelocity for uniformly bringing the strip temperature into the solutionheat treating zone. Inasmuch as the auxiliary unit 152 is adapted to beinserted into the line shown in Figure 3, heated air can be tapped fromthe duct 97 and passed through a blower 154, a high temperatureelectrical resistance heater 155 and into the nozzle manifold 156through a duct 157. A thermostat 1158 is located at the discharge end ofthe duct 157 and is connected into the circuit of the heater 155 wherebythe amount of current supplied thereto is automatically controlled.Thus, the temperature of the strip can be evenly raised to the requiredtemperature without danger of overheating the edges thereof.

In accordance with the provisions of the patent statutes, We haveexplained the principle and operation of our invention and haveillustrated and described what We consider to representthe bestembodiment thereof. However, we desire to have it understood that withinthe scope of the appended claims, the invention may be practicedotherwise than as specifically illustrated and described.

We claim:

1. In the method of continuously heat treating aluminum alloy of theprecipitation hardenable class and in the form of endless strip thesteps comprising rapidly heating said strip in a first heating zone to atemperature within the solution heat treating range, maintaining thetemperature of saidstrip within said range in a second heating zone fora limited period of time greater than that required for initiallyheating the strip and of the order of one-half to three minutes andquenching said strip from a temperature within to a temperaturesubstantially below said' solution. heat treating range.

2. In the method of continuously heat treating aluminum alloy of theprecipitation hardenable class and in the form of endless strip thesteps comprising rapidly heating said strip in a first heating zone to atemperature slightly below the solution heat treating range, applyingadditional heat to said strip in a second heating zone to raise thetemperature thereof to one within the solution heat treating range,maintaining the temperature of said. strip within said range in a thirdheating zone for a limited period of time greater than that required forheating the strip in said first two heating zones and of the order ofone-half to three minutes and quenching said strip from a temperaturewithin to a temperature substantially below said solution heat treatingrange.

3. 1n the method of continuously processing aluminum alloy of theprecipitation hardenable class and in the form of endless strip thesteps comprising rapidly heating said strip in a first heating zone to atemperature within the solution heat treating range, maintaining thetemperature of said strip within said'range'in a second heating zone fora limited period of time greater than that required for initiallyheating the strip and of the order of one-half to three minutes,quenching said stripfrom a temperature within to a temperaturesubstantially below said solution heat treating range and straighteningsaid strip subsequent to the quenching thereof.

4. In the method of continuously heat treating aluminum alloy of theprecipitation hardenable class and in the form of endless strip thesteps comprising rapidly heating the strip in a matter of seconds to atemperature within the solution heat treating range, thereaftermaintaining the strip without interruption of the movement thereof for aperiod of time not less than one-half minute at a temperature within thesolution heat treating range and thereafter immediately and rapidlyquenching the strip.

5. In the method of continuously heat treating aluminum alloy of theprecipitation hardenable class and in the form of endless strip thesteps comprising rapidly and in a matter of seconds heating the strip ina first zone to a temperature slightly below the solution heat treatingrange, applying suthcient additional heat to said strip withoutinterruption of the movement thereof in a second heating zone to raisethe temperature of said strip to one within the solution heat treatingrange, thereafter in a third heating zone maintaining the temperature ofthe strip within the solution heat treating range for a period of timeof not less than one-half minute and upon leaving said last-mentionedzone immediately and rapidly quenching the strip.

6. The method of heat treating aluminum alloy strip or I the like of theprecipitation hardenable class in a heat treating line in which there isprovided in sequence a first strip heating means, strip flatteningmeans, a second strip heating means, and a quenching means and means forcontrolling the tension in the strip during its passage through saidline which consists in rapidly heating the strip in a matter of secondsin said first heating means to a temperature approximately at butslightly less than one within the solution heat treating range,continuously flattening the strip in said flattening means, additionallyheating said strip in said second heating means sulficiently toestablish and maintain the temperature of the strip while passingtherethrough at one within the solution heat treating range for a periodof time of not less than onehalf minute and thereafter immediately andrapidly quenching the strip.

References Cited in the file of this patent UNITED STATES PATENTS1,334,663 MacDonald et al Mar. 23, 1920 1,355,521 Alexander et al. Oct.12, 1920 1,586,897 Harris June 1, 1926 1,607,086 Kinnear Nov. 16, 19261,624,668 Kochendorfer Apr. 12, 1927 1,646,498 Seede Oct. 25, 19271,671,810 Caughey May 29, 1928 1,928,409 Coe Sept. 26, 1933 1,951,874Kellar Mar. 20, 1934 1,972,241 Lorig et al. Sept. 4, 1934 2,060,634 OtisNov. 10, 1936 2,070,833 Keller Feb. 16, 1937 2,083,576 Nock June 17,1937 2,106,178 Keller et al. Jan. 25, 1938 2,174,645 Wetzel Oct. 3, 19392,238,667 Wales Apr. 15, 1941 2,248,185 Nook July 8, 1941 2,278,136 Otiset al Mar. 31, 1942 2,292,511 Ferm Aug. 11, 1942 (References onfollowing page) 9 10 UNITED STATES PATENTS OTHER REFERENCES 2,388,563Nock Nov. 6, 1945 Iron Age, March 8, 1945, pages 58-63. 2,448,009 BakerAug. 31, 1948 Zeerleder: Technology of Light Metals, 1949, published2,448,835 Schefe Sept. 7, 1948 by Elsevier Publishing Co., Inc. N.Y.C.,pages 236, 237, 2,462,202 Kniveton Feb. 22, 1949 5 246, 247, 254257.

2,550,474 Harrington Apr. 24, 1951 Metal Process, Oct. 1951, pages88-92.

1. IN THE METHOD OF CONTINUOUSLY HEAT TREATING ALUMINUM ALLOY OF THEPRECIPITATION HARDENABLE CLASS AND IN THE FORM OF ENDLESS STRIP THESTEPS COMPRISING RAPIDLY HEATING SAID STRIP IN A FIRST HEATING ZONE TO ATEMPERATURE WITHIN THE SOLUTION HEAT TREATING RANGE, MAINTAINING THETEMPERATURE OF SAID STRIP WITHIN SAID RANGE IN A SECOND HEATING ZONE FORA LIMITED PERIOD OF TIME GREATER THAN THAT REQUIRED FOR INITIALLYHEATING THE STRIP AND OF THE ORDER OF ONE-HALF TO THREE MINUTES ANDQUENCHING SAID STRIP FROM A TEMPERATURE WITHIN TO A TEMPERATURESUBSTANTIALLY BELOW SAID SOLUTION HEAT TREATING RANGE.